Chucks are used most often to secure a drilling or boring bit or a screwdriver bit on a motor shaft. A first option for locking the cylindrical shank of a tool in the chuck consists in equipping the chuck with a rack system which, with the aid of a key, locks the jaws on the tool before the machine is used. The rack causes rotation of a sleeve relative to the body, with a nut being mounted on the interior surface of said sleeve, the internal threads of said nut cooperating with the external threads on the jaws. While this solution is satisfactory from a practical standpoint, it has the disadvantage of requiring the use of a key which is frequently misplaced by the user. This use of a key is even more of a constraint in so-called cordless drills, in other words those powered by batteries which are an integral part of the body of the machine, and which lend it considerable mobility. These cordless machines or drills are generally provided with key chucks to hold the tools. This type of chuck permits use in both rotational directions and the passage of an axial screw to lock the chuck to the drill spindle when the rotational direction of the motor shaft tends to favor loosening of the chuck therefrom.
There are chucks which permit satisfactory locking of the tool without requiring the use of a key. A first known solution consists in providing, at the rear end of the sleeve which causes rotation of the nut that drives the jaws, a plurality of teeth. A ring mounted concentrically on the body is kept axially at a distance from the sleeve by the action of a spring and comprises at least one tooth opposite the sleeve. This ring is mounted to rotate freely around the body. In practice, to lock or unlock the tool, the user usually grasps the ring and moves it forward so that the teeth of the ring engage the teeth of the sleeve, at the same time that he sets the body of the chuck rotating, using the motor shaft, in one direction or the other. The result is a relative rotary motion of the matching teeth of the ring and sleeve, producing a series of impacts which promote the locking or unlocking of the jaws, depending on the rotational direction employed.
In another known chuck, the sleeve has at its forward end a part in the shape of a radial collar with a plurality of forward-facing notches. In addition, the screw that drives the jaws has press-fitted onto it a ring comprising forward-facing notches designed to mesh with the notches of the sleeve. A spring abutting the body of the chuck keeps the notches of the ring and sleeve in mesh until a certain force is reached. In practice, it is sufficient for the user who wants to lock or unlock the jaws to keep the sleeve from rotating by using his hand and to operate the motor shaft in one direction or the other. The result is a series of impacts produced by the notches of the ring against the notches of the sleeve, promoting locking or unlocking, as the case may be.
However, these two known chucks have a complicated structure which results in a high manufacturing cost and considerable inconvenience.
Another solution is provided in EP-A-0,300,375 which relates to a chuck using a very fine pitch for the jaws and a series of balls disposed between the nut and the body. The interposition of balls between the nut and the body is already known in industrial chucks. The chuck described in this document has the disadvantage of not having perfect self-unlocking characteristics and of being very awkward to operate for both locking and unlocking, in view of the fineness of the threads, and also of requiring a large outer diameter of the chuck in order to achieve a good locking action. Chucks comprising internal balls suffer from the disadvantage of not being able to be rotated to the left, of not permitting the passage of a central screw to lock the chuck to the spindle of the machine, and of loosening under the influence of powerful vibrations when operating with impacts.